The present invention is related to a filter medium for gaseous fluids, and more particularly related to a nonwoven web highly suitable for gaseous fluid filter media.
Different types of nonwoven webs have been used as filter media for various liquid and gas filtration applications, and such nonwoven webs include meltblown fiber webs, solution spun fiber webs, wet-laid fiber webs, carded fiber webs, air-laid fiber webs and spunbond fiber webs. Of these nonwoven fiber webs, microfiber nonwoven webs, such as meltblown fiber webs, have been widely employed as fine particle filter media since the densely packed fine fibers of these webs provide fine interfiber pore structures that are highly suitable for mechanically trapping or screening fine particles and therefore are conducive for providing a high filter efficiency. However, the fine pore structure of meltblown fiber webs and other similar webs having densely packed fine fibers results in a low permeability, creating a high pressure drop across the webs. Consequently, the low permeability of fine fiber filter media requires the application of a high driving pressure to establish an adequate throughput rate across the filter media. Furthermore, as contaminants accumulate on or in the surface of the filter media, the contaminants quickly clog the small interfiber pores, further reducing the permeability of the media, increasing the pressure drop across the media and rapidly shortening their service-life. In contrast, filter media with large interfiber pores and thus a high permeability, which contain sparsely packed and typically thick fibers, require a relatively low driving pressure to provide an adequate throughput rate and extended service-life. However, highly permeable filter media, e.g., fiberglass HVAC filters, suffer from a low filter efficiency in that the large interfiber pore structures of the media do not provide interstitial configurations that are suitable for entrapping fine contaminant particles.
In addition to the low permeability, microfiber webs typically do not have high physical strength. Thus, in general, microfiber filter media are laminated to at least one separate self-supporting layer, e.g., a spunbond fiber web, adding cost and complexity to the manufacturing process. For example, U.S. Pat. No. 4,589,894 to Gin et al. discloses a vacuum cleaner filter bag that is produced from a laminate of a fine fiber nonwoven layer and two highly porous nonwoven support layers.
Although filter media produced from nonwoven webs of coarser fibers, such as spunbond fiber webs and commonly-available staple fiber webs, have been used in filtration applications such as stove hood filters, they have not been used as filter media for fine particles. This is because, as mentioned above, the thickness of these coarse fibers causes the nonwoven webs to have larger interfiber pores than those of microfiber webs at a given basis weight, and consequently, nonwoven webs of spunbond fibers and staple fibers have not been used in fine particles filtration applications.
There is a need for self-supporting filter media that provide combinations of desirable filtration properties, including high filtration efficiency, high permeability, low pressure drop, high throughput, long service-life and self-supporting strength.